Here is quick fix for an issue where AllItems view was set to blank and was set to hidden.  Here is how I fixed it.  Nothing fancy just some old Server OM calls in PowerShell.

First you have to list all the views to find it out what’s going wrong and put it in a list.

$SiteUrl = https://portal.contoso.com     
$ListUrl = "Discussions List"            

$Databases = @();
       
$Web = Get-SPWeb -Identity $SiteUrl            
if ($Web -ne $null)            
{
    $List = $Web.Lists[$ListUrl]
    foreach($view in $List.Views)
    {
        $ViewFile = $targetUrl.GetFile($view.Url)
        $Viewobj = New-Object PSObject
        Add-Member -input $Viewobj noteproperty 'ID'  $view.ID
        Add-Member -input $Viewobj noteproperty 'Title'  $view.Title
        Add-Member -input $Viewobj noteproperty 'CreatedBy'  $ViewFile.Author
        Add-Member -input $Viewobj noteproperty 'Modified' $ViewFile.TimeLastModified 
        Add-Member -input $Viewobj noteproperty 'ModifiedBy' $ViewFile.ModifiedBy 
        Add-Member -input $Viewobj noteproperty 'CreatedOn' $ViewFile.TimeCreated  
        Add-Member -input $Viewobj noteproperty 'Private' $view.PersonalView  
        Add-Member -input $Viewobj noteproperty 'Url' $view.Url  
        Add-Member -input $Viewobj noteproperty 'Hidden' $view.Hidden  
        $Databases += $Viewobj
          
    }

    #Export If Needed.        
    #$Databases  | Out-File C:\View.txt

}
#Select the View and Click OK in GridView

$aBadView= $Databases | Out-GridView -PassThru
$aBadView.ID

Now You have the View ID you have do it like below

$ID = [guid]("0b57d149-dbcc-4652-b72f-2fa024238301")    #$ID = [guid]($aBadView.ID)
$BadView = $List.Views[$ID]
$BadView.Title = "Subject"
$BadView.Hidden = $false
$BadView.Update()

Run the first part of script again to verify.